This invention relates to containers, and is particularly concerned with a method of manufacture of containers that resemble tins with press-in lids which can be levered off using the rim of the container as a fulcrum. These have traditionally been produced in metal, and have been widely used for such contents as paint, drinking chocolate and cornflour.
Versions in plastics material have been manufactured, and hitherto they have been produced by welding together two discrete mouldings. The reason for this two-piece construction is the inward overhang of the top flange or rim. If known injection moulding methods are employed, it has been considered impossible to arrange it so that the inside male core can be extracted. The opening is too small to permit any form of collapsible insert. Blow moulding might offer a solution, but the problem with the rim has not been solved.
It is the aim of this invention to overcome this difficulty and to enable such a container to be made by blow moulding.
According to one aspect of the present invention there is provided a method of making a container to which entry is via an opening inset from adjacent container walling, the material of the container being susceptible to deformation when heated, wherein in a first stage an embryo container is formed with a rim defining an opening projecting outwardly from what is to become the container walling, and wherein in a second stage that embryo container walling, at an elevated temperature, is urged outwardly while the rim is held, whereby the walling expands to leave the inner part of the rim inset therefrom.
It will be assumed hereafter that both the embryo and finished container are in an upright attitude with the opening uppermost.
The formation of the embryo container in the first stage is conveniently by injection moulding using an inner mould core which is subsequently extractable through said opening, while the transformation from embryo to finished container in the second stage is preferably by blow moulding via the opening, although at least some of the distortion or stretching inherent in the process could be achieved mechanically.
In the preferred form, the opening of the embryo container widens towards the mouth, locally thinning the container walling. The rim may then include a lateral flange projecting outwardly from the portion of thinned walling, this thinned portion being deformed in the second stage to co-operate with the underside of the flange. The embryo container also preferably has a further flange on the lateral flange, projecting downwardly to surround the opening end of the container, and the deformation of the thinned portion extends to it being forced against the inside of the further flange and to step out below its free edge.
With this arrangement, and if the material of the container is transparent, a data band can be provided on the inside of the further flange of the embryo container to be legible therethrough, the band being made captive by the deformed thinned portion.
Conveniently, at the root of the lateral flange there is an upstanding rib which internally smoothly continues the contour of the mouth of the opening. Also, at the outer edge of the lateral flange there may be, on the upper side, a stepped extension forming with said rib an upwardly open channel around the opening. The stepped extension may project laterally beyond the flanges, its underside providing means for locating the embryo container during the blow moulding.
According to another aspect of the present invention there is provided a container made by the method outlined above.